refactor(mail): extract the mu4e attachment workflow into its own module

The attachment-save UI (the MIME-part filters, the three save commands, and the `special-mode'-derived selection buffer) was ~230 lines in `mail-config.el' and depended on nothing else there. It moves to `modules/mu4e-attachments.el', which `mail-config' now requires. `cj/email-map' and its C-; e bindings stay put. The keymap just points at commands that now live next door.

The unit tests move with it: `test-mail-config-attachments.el' becomes `test-mu4e-attachments.el' and requires the new module directly instead of pulling in the whole mu4e and org-msg use-package stack. The two tests that check `cj/email-map' wiring move to a new `test-mail-config.el', since that map belongs to `mail-config'. One of the moved tests quietly relied on a real mu4e install (it loaded `mu4e-mime-parts' through a load-path entry that loading `mail-config' happens to add), so it now stubs that path itself.

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